Kaan Inal

A review on the effect of rare-earth elements on texture evolution during processing of magnesium alloys

Magnesium, the lightest structural metal, is approximately four times lighter than steel—the most widely used metal in industrial applications. Currently available Mg alloys, however, are impractically expensive for use in automotive structural components, as severe ductility problems require forming operations at elevated temperatures and an exclusion from critical safety components. With a strong impetus in research having sprung up during the last two decades, addition of rare-earth elements in small quantities emerged as a potential solution for simultaneously delivering the ductility and weight requirements for automotive applications. These improvements are arguably achieved by virtue of texture weakening and enhancement of non-basal slip. However, ways by which rare-earth elements modify texture remain very elusive, and no consensus on the driving mechanisms has been reached in the literature as of yet. We take a look back at different paradigms held for the action of rare-earth additions, and examine key facts that may reconcile controversies. We attempt to identify critical gaps and suggest venues to overcome them. These gaps, once filled, may promote Mg alloys to become a stronghold for lightweighting, which will exceptionally benefit our environment and wellbeing.

Crystal plasticity based finite element modelling of large strain deformation in AM30 magnesium alloy

In this paper, the finite strain plastic deformation of AM30 magnesium alloy has been simulated using the crystal plasticity finite element method. The simulations have been carried out using a rate-dependent elastic–viscoplastic crystal plasticity constitutive model implemented in a user defined material subroutine (UMAT) in the commercial software LS-DYNA. The plastic deformation mechanisms accounted for in the model are the slip systems in the matrix (parent grain), extension twinning systems and the slip systems inside the extension twinned regions. The parameters of the constitutive model have been calibrated using the experimental data. The calibrated model has then been used to predict the deformation of AM30 magnesium alloy in bending and simple shear. For the bending strain path, the effects of texture on the strain accommodated by the deformation mechanisms and bending moment have been investigated. For simple shear, the effects of texture on the relative activity of deformation mechanisms, shear stress and texture evolution have been investigated. Also, the effect of twinning on shear stress and texture evolution has been studied. The numerical analyses predicted a more uniform strain distribution during bending and simple shear for rolled texture compared with extruded texture.

Simulations of Forming Limit Diagrams for the Aluminum Sheet Alloy 5754CC

In this paper, the capability of the four different yield functions to predict forming limit diagrams of continuous cast AA‐5754 Aluminum sheet have been compared with focus on the differences in the predicted limit strains based on the method of determining the yield function parameters that do not employ a linear transformation tensor on the stress tensor. The yield functions proposed by Hill (1948, 1990 and 1993) and Barlat (1989), which have been successfully used to predict material anisotropy in aluminum alloys in the literature, have been considered in this study. The forming limit diagrams (FLDs) have been calculated numerically based on these yield functions together with the Marciniak‐Kuczynski (M‐K) approach.

Numerical Modeling of Second-Phase Particle Effects on Localized Deformation

A new finite element analysis based on rate dependent crystal plasticity theory has been developed to investigate the effects of second-phase particles on the initiation and propagation of localized deformation in the form of shear bands. The new model can incorporate electron backscatter diffraction data into finite element analyses. The numerical analysis not only accounts for crystallographic texture (and its evolution) but also accounts for grain morphologies. A unit-cell approach has been adopted where an element or a number of elements of the finite element mesh are considered to represent a single crystal within the polycrystal aggregate. Second-phase particles in the form of finite elements with stiff elastic properties are randomly distributed within the unit cell. Numerical simulations of unixial tension, in-plane plane strain tension, and balanced biaxial tension have been performed by models with and without second-phase particles for a direct chill-cast AA5754 aluminum alloy sheet. The effects of various parameters, such as second-phase particle distribution, texture evolution, and strain paths on particle induced localized deformation patterns, are also investigated.

Determination of the Minimum Scan Size to Obtain Representative Textures by Electron Backscatter Diffraction

A new method for analyzing microstructure is proposed to evaluate the long-range dependence of texture. The proposed method calculates the average disorientation as a function of distance between data points as measured by electron backscatter diffraction patterns. This method gives a measure of clustering of texture and is used to evaluate accurately the effective grain size. This procedure in conjunction with Information theory is used to estimate a representative scan size for various materials. Analyses show that the optimal scan size depends on grain morphology and crystallographic texture. The results also indicate that on an average the optimal scan size needs to be 10 times the effective grain size.

Orientation and Path Dependence of Formability in the Stress- and the Extended Stress-Based Forming Limit Curves

This article analyzes the formability data sets for aluminum killed steel (Laukonis, J. V., and Ghosh, A. K., 1978, “Effects of Strain Path Changes on the Formability of Sheet Metals ,” Metall. Trans. A., 9, pp. 1849–1856), for Al 2008-T4 (Graf, A., and Hosford, W., 1993, “Effect of Changing Strain Paths on Forming Limit Diagrams of Al 2008-T4 ,” Metall. Trans. A, 24A, pp. 2503–2512) and for Al 6111-T4 (Graf, A., and Hosford, W., 1994, “The Influence of Strain-Path Changes on Forming Limit Diagrams of Al 6111 T4 ,” Int. J. Mech. Sci., 36, pp. 897–910). These articles present strain-based forming limit curves (ϵFLCs) for both as-received and prestrained sheets. Using phenomenological yield functions, and assuming isotropic hardening, the ϵFLCs are transformed into principal stress space to obtain stress-based forming limit curves (σFLCs) and the principal stresses are transformed into effective stress and mean stress space to obtain the extended stress-based forming limit curves (XSFLCs). A definition of path dependence for the σFLC and XSFLC is proposed and used to classify the obtained limit curves as path dependent or independent. The path dependence of forming limit stresses is observed for some of the prestrain paths. Based on the results, a novel criterion that, with a knowledge of the forming limit stresses of the as-received material, can be used to predict whether the limit stresses are path dependent or independent for a given prestrain path is proposed. The results also suggest that kinematic hardening and transient hardening effects may explain the path dependence observed in some of the prestrain paths.

Reconstruction of the 3D representative volume element from the generalized two-point correlation function

This paper presents the first application of three-dimensional (3D) cross-correlation microstructure reconstruction implemented for a representative volume element (RVE) to facilitate the microstructure engineering of materials. This has been accomplished by developing a new methodology for reconstructing 3D microstructure using experimental two-dimensional electron backscatter diffraction data. The proposed methodology is based on the analytical representation of the generalized form of the two-point correlation function—the distance-disorientation function (DDF). Microstructure reconstruction is accomplished by extending the simulated annealing techniques to perform three term reconstruction with a minimization of the DDF. The new 3D microstructure reconstruction algorithm is employed to determine the 3D RVE containing all of the relevant microstructure information for accurately computing the mechanical response of solids, especially when local microstructural variations influence the global response of the material as in the case of fracture initiation.

Small-angle X-ray scattering investigation of deformation-induced nanovoids in AA6063 aluminium alloy

Abstract Small-angle X-ray scattering studies of microstructure in metallic systems are prone to contamination by double Bragg scattering from the crystalline matrix. This is particularly problematic to the study of fracture in ductile metals via the nucleation and growth of nanovoids in response to plastic deformation. We show clear evidence of the presence of these scattering artefacts in the scattering data from representative Al systems and describe a simple method of numerically isolating and removing potentially misleading information to reveal the true small angle scattering response of the sample. This data correction process is used to obtain quantitative measurements of the nanovoid volume fraction in deformed AA6063. The SAXS results yield values comparable to existing predictions of the total vacancy volume fraction obtained from the mechanical stress–strain data.

Constitutive Analyses of Tensile Tests on Pre-Rolled AA3003 Sheets

A new rationale to assess the work-hardening locus for pre-rolled sheets is described based on the realization that since the internal stresses necessarily sum to zero, the mean dislocation density remains the same upon re-pull in the rolling direction. Thus the 0.2 % yield stress as function of thickness strains results in an estimate of the stress-strain relation during rolling. Under plane strain, the thickness strain is negative to that of extension and hence the deduced rolling locus is compared to that of extrapolated tensile one of the start sheet. This comparison indicates that the onset of Stage IV occurs when volume fraction of point defects produced attains about 2 %.

New Methodology to Determine Stable Texture Components under Cold Rolling in FCC Metals

In this work evolution of texture components during deformation of AA5754 aluminum alloy sheet under cold rolling is studied by analyzing the evolution of element-rotation-distribution calculated using a rate-dependent crystal plasticity finite element model (CPFEM). The proposed criteria can successfully predict the stability of a given textural component for cold rolling deformation in FCC materials with high stacking fault energy that deform predominantly by slip. Comparison of simulation results with experimental data shows that this approach successfully captures the stable textures reported for cold rolled AA5754 sheets. With the initial texture and the strain path, it is believed that, the method described in this work can be used to predict the final stable textures without any need for expensive crystal plasticity based numerical simulations and this could be of immense help for simulating large strain deformation of macroscale sheet samples exhibiting texture evolution.